Lack of information on the immunologic mechanisms responsible for protection against HIV infection remains one of the major obstacles to the development of a safe and effective AIDS vaccine. Vaccination of macaques with attenuated SIV strains has consistently proven to be the most effective means to induce protection against pathogenic SIV challenge in macaques. Intensive study of macaques vaccinated with attenuated SIV strains therefore represents one of the best experimental models available for the determination of mechanisms of protective immunity against lentivirus infection. The overall goal of this Program Project application is to undertake a comprehensive, multidisciplinary effort to define mechanisms of immune protection mediated by live attenuated SIV strains. Complementary experiments conducted by three principal investigators with distinct areas of expertise will examine: 1. Mechanisms of mucosal protection induced by attenuated SIV. These experiments will undertake a detailed examination of the evolution of adaptive and innate immune responses induced by SIVAnef and correlate these responses with protection, examine the effect of prolonged B cell depletion on protective immunity, and study viral replication and immune responses in the female reproductive tract of SIVAnefvaccinated animals after vaginal challenge. 2. The contribution of anti-envelope immune responses to protection mediated by live attenuated SIV. Specific questions include: Does a mismatch of envelope sequences in the challenge virus decrease the degree of protection? Does challenge with a closely-matched SIV strain that differs dramatically in coreceptor usage influence the degree of protection? Does variation in the strength of the anti-envelope antibody response induced using modified single-cycle SIV influence the degree of protection? 3. Mucosal immunity and heterologous protection induced by single-cycle SIV (scSIV). These experiments will examine if the site of immunization with scSIV determines the mucosal homing properties of T cell responses and resistance to an intrarectal challenge with SIVmac239;whether the site of priming influences the ability of virus-specific T cell responses to protect against a vaginal challenge with SIVmac239;and whether immunization with a mixture of antigenically diverse strains of scSIV can broaden virus-specific immune responses and enhance protection against a heterologous challenge with SIVmac239. Results from these studies should shed light on the nature of immune responses able to protect against HIV/SIV infection, which remains one of the outstanding unanswered questions of AIDS vaccine research, and thus will have important implications for the design of clinically applicable AIDS vaccines.